The use of dichlorodifluoromethane (CFC12) for refrigerators and automotive air conditioners has recently been legally regulated to protect the ozone layer, and will eventually be totally banned. The use of chlorodifluoromethane (HCFC22) for air conditioners will be regulated. Thus, hydrofluorocarbons (HFC) which do not destroy the ozone layer have been developed as substitutes for CFC12.
However, since the polarity of hydrofluorocarbons is higher than that of CFC12, the use of a conventional lubricating oil which has commonly been used as a refrigeration oil, such as naphthenic mineral oil, poly .alpha.-olefin or alkylbenzene, causes two-layer separation at a low temperature due to the poor compatibility between these lubricating oils and hydrofluorocarbons. Two-layer separation hampers oil return, which in turn interferes with heat transfer due to the deposition of a thick oil film around the condenser and evaporator as heat exchangers. It can also cause failures due to poor lubrication and foaming during the starting operation. Therefore, the conventional refrigeration oils cannot be used in the presence of these new refrigerants.
As for lubricity, CFC12 generates hydrogen chloride upon partial decomposition, which reacts on the friction surface to form a chloride coating, thereby improving its lubricity. On the other hand, the hydrofluorocarbons cannot be expected to have such an effect because they do not contain a chlorine atom, and therefore, the refrigeration oils used in combination with hydrofluorocarbons are required to have better lubricity than that of the conventional refrigeration oils.
In addition, the refrigeration oils used in combination with hydrofluorocarbons need to have good thermal stability in the presence of hydrofluorocarbons.
Moreover, since organic substances are present in compression refrigerating machines for electric refrigerators, for example, materials for motor components such as insulators and enameled wires, the working fluid comprising a hydrofluorocarbon and a refrigeration oil must not adversely affect these organic materials and must have a good insulating property.
Polyether compounds which can be used as refrigeration oils in combination with hydrofluorocarbons such as 1,1,1,2-tetrafluoroethane (HFC134a) are disclosed in U.S. Pat. No. 4,755,316, Japanese Patent Laid-Open Nos. 198694/1989, 256594/1989, 259093/1989, 259094/1989, 259095/1989, 84491/1990, 102296/1990, 129294/1990, 132176/1990, 132177/1990, 132178/1990, 132179/1990, 173195/1990, 180986/1990, 180987/1990, 182780/1990, 242823/1990, 242888/1990, 258896/1990, 269195/1990, 276880/1990, 276881/1990, 272097/1990, 281098/1990, 305893/1990, 14894/1991, 28296/1991, 33192/1991, 33193/1991 and other publications.
Since the polyether compounds have a polarity higher than that of naphthenic mineral oils, their compatibility with HFC134a at a low temperature is very good. However, polyether compounds cannot be safely used as refrigeration oils, since they pose the problem of two-layer separation upon a rise in temperature, as stated in U.S. Pat. No. 4,755,316.
The polyether compounds involve other problems. One of them is that they have a poor insulating property. This is an extremely important problem which makes it impossible to use the polyether compounds for refrigerating machines used in electric refrigerators. Another problem is that they have high hygroscopicity. Due to the moisture in the polyether compound, the thermal stability in the presence of HFC134a is degraded, causing organic materials such as PET films to be hydrolyzed.
In order to eliminate the above-mentioned problems in the polyether compounds such as insulating property, hygroscopicity, etc., various ester compounds have been developed. For instance, as refrigerating oils which can be used together with 1,1,1,2-tetrafluoroethane (HFC134a), mixed oils of polyether oils and ester oils are disclosed in U.S. Pat. No. 4,851,144 (corresponding to Japanese Patent Laid-Open No. 276894/1990) and Japanese Patent Laid-Open No. 158693/1990; and ester oils are disclosed in U.S. Pat. No. 2,261,541 and European Patent No. 406,479.
The ester compounds are excellent in compatibility with the hydrofluorocarbons, and they are also excellent in thermal stability in the presence of the hydrofluorocarbons. In addition, when compared with the polyether compounds, the ester compounds have a remarkably excellent insulating property and a considerably low hygroscopicity.
However, hydrofluorocarbon-ester oils are more likely to absorb moisture because of the increased polarity of hydrofluorocarbon and oil in comparison with CFC12-mineral oils, which constitute conventional working fluids, and the water which remains unremoved can hydrolyze the ester to form carboxylic acid, which may in turn corrode metals to cause wear.
As stated above, the presently developed hydrofluorocarbonpolyether oils have problems in hygroscopicity and insulating property, and the hydrofluorocarbon-ester oils have problems in hydrolysis resistance. In addition, when compared with the conventional CFC12-mineral oils, they are more likely to absorb water, which may cause a decrease in thermal stability and the deterioration of the organic materials, which may in turn corrode metals and cause wear. Therefore, these oils do not meet the requirement of a working fluid for refrigerating machines.